1. Field of the Invention
The present disclosure relates generally to a channel estimation method and apparatus of a terminal in a cellular mobile communication system including a plurality of base stations and, more particularly, to a method and an apparatus for estimating channel efficiency in a Cooperative Multi-Point (CoMP) system supporting cooperative downlink transmission from a plurality of base stations to a terminal.
2. Description of the Related Art
Mobile communication systems have evolved into high-speed, high-quality wireless packet data communication systems that provide data and multimedia services beyond the early voice-oriented services. Recently, various mobile communication standards, such as High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA), Long Term Evolution (LTE), and LTE-Advanced (LTE-A) defined in 3rd Generation Partnership Project (3GPP), High Rate Packet Data (HRPD) defined in 3rd Generation Partnership Project-2 (3GPP2), and 802.16 defined in IEEE, have been developed to support the high-speed, high-quality wireless packet data communication services.
Particularly, LTE is a communication standard developed to support high speed packet data transmission and to maximize the throughput of the radio communication system with various radio access technologies. LTE-A is an evolved version of LTE that improves data transmission capability.
The existing 3rd generation wireless packet data communication systems (including, for example, HSDPA, HSUPA, and HRPD) adopt Adaptive Modulation and Coding (AMC) and channel-sensitive scheduling techniques to improve transmission efficiency.
In a wireless packet data communication system that adopts AMC, the transmitter is capable of adjusting the data transmission amount based on the channel condition. Specifically, the transmitter decreases the data transmission amount for bad channel conditions so as to fix the received signal error probability at a certain level, and increases the data transmission amount for good channel conditions so as to transmit a large amount of information efficiently while maintaining the received signal error probability at an intended level.
In a wireless packet data communication system that adopts channel-sensitive scheduling, the transmitter serves a user having good channel conditions first among a plurality of users so as to increase the system capacity, as compared to allocating a channel to one user. Such an increase of system capacity is referred to as multi-user diversity gain.
When using AMC along with a Multiple Input Multiple Output (MIMO) transmission scheme, it may be necessary to take a number of spatial layers and ranks for transmitting signals into consideration. The transmitter determines the optimal data rate in consideration of the number of layers for use in MIMO transmission.
In general, OFDMA is expected to provide superior system throughput as compared to CDMA. One of the main factors that allows OFDMA to increase system throughput is frequency domain scheduling capability. As channel sensitive scheduling increases the system capacity using a time-varying channel characteristic, OFDM can be used to obtain more capacity gain using a frequency-varying channel characteristic.
Research has been conducted to replace CDMA, used in 2nd and 3rd legacy mobile communication systems, with OFDMA for the next generation wireless communication system. Standardization of 3GPP and 3GPP2 is being conducted for an OFDMA-based evolved system.
FIG. 1 is a diagram illustrating a structure of a radio frame of an LTE-A system.
Referring FIG. 1, a radio frame consists of 10 subframes, and each subframe consists of two slots. The subframes constituting one radio frame are designated by indices 0 through 9, and the slots are designated by indices 0 through 19 (#0˜#19).
FIG. 2 is a diagram illustrating a cellular mobile communication system in which the transmit/receive antennas are arranged at the center of the cells.
Referring to FIG. 2, in the cellular mobile communication system composed of a plurality of cells, a User Equipment (UE) receives a mobile communication service from a cell selected in a semi-static duration using the above described techniques. The cellular mobile communication system may include three cells 100, 110, and 120. The cell 100 serves UEs 101 and 102 within its service area, the cell 110 serves UE 111, the cell 120 serves UE 121, and reference numbers 130, 131, and 132 denote the evolved Node Bs (eNBs).
The UE 102 served by the cell 100 is located far from the eNB 130 as compared to the UE 101. The UE 102 experiences significant interference from the central antenna of the neighbor cell 120 and is served by the UE 100 at relatively low data rate.
When the cells 100, 110, and 120 provide the wireless communication services independently, they transmit a Reference Signal (RS) for downlink channel estimation. Particularly, in a 3GPP LTE-A system, the UE measures the channel condition between the eNB and itself using a Cell-specific Reference Signal (CRS) or a Channel Status Information Reference Signal (CSI-RS) transmitted by the eNB.
In the case of the cellular mobile communication system shown in FIG. 2, the UE located at the cell edge suffers significant interference from the neighbor cell and thus cannot be served at a high data rate. Specifically, in the cellular mobile communication system configured as shown in FIG. 2, the ability to allocate the data rate required for providing the UEs with high speed data service within the cell is influenced by UE location within the cell. Accordingly, the conventional cellular mobile communication system is capable providing the UE close to the center of the cell with the data service at a higher data rate, but cannot guarantee the high data rate to the UE located far from the center of the cell. In order to provide the UE located at the cell edge with the data service at a high data rate, a new transmission technique, CoMP, has been proposed for multiple cells to perform transmission to a UE cooperatively in the LTE-A system. In the case of using CoMP transmission technique, multiple CSI-RSs for different cells may be allocated per UE, and thus the UE has to receive the multiple CSI-RSs to estimate multiple channels efficiently.